In the prior art, rubber hoses were generally used in a wide variety of uses as automobile fuel feed hoses, torque converter hoses, power steering hoses, air conditioner hoses, refrigerator hoses, propane gas feed hoses, hydraulic hoses as well as household utility. A variety of media including water, oil, organic solvents, and gases are conducted through the rubber hoses. Therefore, the rubber hoses are required not only to be flexible, but also to be resistant to these transfer media and fully impermeable to organic gases and organic solvents.
In general, rubber surfaces do not always exhibit high resistance against chemical loads such as organic gases and organic solvents. Prior art approaches for improving the organic gas and solvent resistance of rubber included (1) to use a rubber material having different polarity from the solvent, (2) to increase the degree of crosslinking of rubber, (3) to increase the amount of filler added, and (4) to increase the amount of rubber used, that is, wall thickness. Approach (1) requires expensive rubbers such as chloroprene rubber, butadieneacrylonitrile rubber, acryl rubber, epichlorohydrin rubber, and fluororubber. Approach (2) detracts from softness and flexing resistance of rubber. Approach (3) is limited in conjunction with processability and physical properties. The solvent resistance is improved only in proportion to a volume fraction of the filler. As to approach (4), an increased wall thickness adds to product weight and cost and detracts from softness. In addition to these problems, either of these approaches fails to increase the organic gas and solvent resistance of rubber beyond a certain limit.
In order to enhance the performance and the gas and solvent resistance of rubber hoses, a number of proposals were made for composite resin-rubber hoses. Typically a thin resin layer is applied to the inside surface of a rubber hose to be in contact with a transfer medium. In the composite structure, the resin layer serves such functions as organic solvent resistance, gas impermeability, and chemical resistance while the outer rubber layer is responsible for hose's own functions such as softness and vibration absorption.
Composite rubber hoses are typically manufactured by coating a solvent soluble nylon film to the inside surface of a rubber hose as disclosed in Japanese Patent Application Kokai No. 113885/1985. Alternative methods are disclosed in Japanese Patent Publication Nos. 45302/1988 and 125885/1988 wherein a rubber hose is manufactured by extrusion coating a resin on a mandrel or shaping core for the hose to form a resin film. An adhesive is applied to the resin film. After drying, an intermediate rubber composition is coated, a reinforcing layer is braided, and an overcoating rubber layer is further coated. A final vulcanization shaping step completes a hose having improved organic gas and solvent resistance.
In the case of composite rubber hoses for accommodating coolant fluid for automobile air conditioners or the like, nylon resins are most often used as the resin forming the inner layer of the hose. A variety of nylon resins are used in practice including nylon 6, nylon 66, nylon 6/nylon 66 copolymer, nylon 11, nylon 12, nylon 4, copolymers thereof, modified ones, blends thereof, and blends of nylons and olefins.
The nylon used on the inner surface of rubber hoses for accommodating coolant fluid has to meet the following requirements.
(1) It is low permeable to Freon gas (gas impermeability). PA1 (2) It is not susceptible to hydrolysis by moisture penetrating from without the hose (moisture resistance). PA1 (3) It is resistant to heat. PA1 (4) It withstands dynamic motion as in an impulse test. PA1 (5) It is gas tight at a base or connector end.
With respect to these requirements, nylon 6, nylon 66, and nylon 6/nylon 66 copolymers are very low in Freon gas permeability, but relatively high in moisture permeability. In turn, nylon 11, nylon 12 and analogs are low in moisture permeability and less susceptible to hydrolysis, but moderately high in Freon gas permeability.
Some of these drawbacks may be eliminated by substituting a butyl rubber or Hypalon (chlorosulfonated polyethylene by E. I. duPont) having low moisture permeability for the rubber portion. The use of butyl rubber can reduce moisture permeation from the exterior. However, if the nylon layer contributing to Freon impermeability fails due to the presence of defects therein, such a failure would cause a serious failure of the entire hose because the butyl rubber layer is not self sustaining. In turn, the use of Hypalon is limited because it is expensive and difficult to bond.
Attempts have been made to blend nylon 6 or nylon 66 or a copolymer thereof having Freon gas impermeability with nylon 11 or nylon 12 having low moisture permeability or with olefin resins. These attempts improve moisture impermeability and hydrolysis resistance at the sacrifice of Freon gas impermeability.
Therefore, prior art combinations of rubber with nylons or the like were unsuccessful in fully satisfying the requirements of rubber hoses for transfer of coolant and similar fluids. It was thus proposed to wind a length of aluminum or copper strip or a length of plastic tape having such a metal thin film coated thereon around the outer periphery of a resinous inner tube, followed by coating a rubber layer thereon, or to form the inner surface of a rubber hose directly from a length of tape having a metal thin film coated thereon (see Japanese Utility Model Application Kokai Nos. 177620/1979, 198221/1979, 162379/1981, 94975/1983, 99582/1983, and 158879/1983, Japanese Patent Application Kokai No. 205144/1982, and Japanese Patent Publication No. 13812/1988).
These proposals take advantage of the barrier nature of a metal film to provide Freon and moisture impermeability, but suffer from various problems. Manufacture of these structures is complicated and thus expensive. When a length of metal foil or metallized tape is used, the seam between adjoining turns is simply a physical overlap. The turns can be displaced to form a gap therebetween under severe service conditions, resulting in a loss of the barrier nature of the metal thin film so that gas leakage can occur. Reduced softness is also a problem of these rubber hoses. Further, these methods generally use an adhesive in applying rubber on a metal foil or metallized tape wrap on a resinous inner tube to form a composite structure. The adhesive will undesirably lose its adhesive force upon contact with an organic solvent that has penetrated through the resinous inner tube.
To overcome these prior art problems, we previously proposed to coat a synthetic resin inner tube on the outer surface with a metal or metal compound by dry plating, as disclosed in Japanese Patent Application Kokai Nos. 209224/1990 and 209225/1990 (which corresponds to U.S. Pat. No. 5,271,977).
One of the important features of rubber hoses is flexibility because the rubber hoses are often bent angularly during service. When a rubber hose having an outer diameter of 20 mm and an inner diameter of 10 mm is rounded to a diameter of 100 mm, a strain of about 10% is applied to the hose at the inner surface. According to our study, strains of about 10% would induce cracking and crazing in general metals and metal compounds even in thin film form, which would thus lose their barrier nature. Although these metals and metal compounds are acceptable if hose service conditions such as bending radius are restricted, rubber hoses featuring flexibility are required to maintain gas impermeability even under extremely angular bends.